Numerous catalyst are known for the hydrogenation of compounds containing unsaturated double bonds, catalysts which may be classified into two groups:
(1) Heterogeneous catalysts, generally consisting of a metal such as Ni, Pd, Pt, Ru, etc., optionally deposited on a support such as carbon, silica, alumina, calcium carbonate, etc.; and PA1 (2) homogeneous catalysts such as (a) Ziegler catalysts consisting of a combination of an organic salt of Ni, Co, Fe, Cr, etc. and a reducing agent such as for instance organoaluminum compounds, and (b) single component organometallic compounds of Ru, Rh, Ti, La, etc. PA1 (a) a titanium compound of the formula ##STR3## (I) wherein A.sub.1 represents an optionally substituted indenyl group of the formula ##STR4## wherein the substituents R.sub.1 and R.sub.2 may be the same or different and each may be selected from halogen, phenyl which optionally may bear one or more of the same or different substituents, lower alkyl, lower alkoxy, phenoxy, phenylalkoxy, benzyl and a bulky substituent containing one or more hetero atoms such as tri (loweralkyl)silyl, --NPh.sub.2, --NHPh, --BPh.sub.2, and --B(OPh).sub.2, wherein n may be an integer of from 0 to 4, preferably from 0 to 2 and more preferably from 0 to 1, and m may be an integer of from 0 to 3, preferably from 0 to 2 and more preferably from 0 to 1, and PA1 (b) an alkali metal hydride, added as such or prepared in situ in the polymer solution from the alkali metal terminated living polymer and/or from additionally added alkali metal alkyl.
Heterogeneous catalysts are widely used in industry, but compared with homogeneous catalysts they are less active and hence, in order to carry out the desired hydrogenation with these heterogeneous catalysts, large quantities of catalyst are needed and the reaction must be carried out at relatively high pressures and temperatures. The homogeneous catalysts are generally more active. A small amount of catalyst is sufficient and the hydrogenation reaction can be carried out under milder pressure and temperature conditions.
Polymers of conjugated dienes such as 1,3-butadiene and isoprene and the copolymers of these dienes with vinylaromatic monomers, e.g. with styrene, are widely used in industry as elastomers. These polymers contain double bonds in their chain, which permit their vulcanization, but whose presence causes a low resistance to ageing and oxidation. Some block copolymers of conjugated dienes and vinylaromatic hydrocarbons are used without vulcanization as thermoplastic elastomers, as transparent impact-resistant resins, or as modifiers or compatibilizers of polystyrene and polyolefin resins. However, these copolymers have a low resistance to ageing and oxidation by atmospheric oxygen and by ozone, due to the presence of double bonds in their chain. Hence, the use of these copolymers in applications requiring exposure to the external environment is limited. The resistance to oxidation by oxygen and ozone, and, in general, the resistance to ageing, may be considerably improved by hydrogenating these polymers to obtain total or partial saturation of the double bonds. Numerous processes have been proposed for the hydrogenation of polymers which contain olefinic double bonds. Two types of processes are generally involved: those which use the aforementioned supported heterogeneous catalysts and those using homogeneous catalysts of the Ziegler type or organometallic compounds of rhodium and titanium.
In the processes using supported heterogeneous catalysts, the polymer to be hydrogenated is first dissolved in a suitable solvent and then contacted with hydrogen in the presence of the heterogeneous catalyst. The contact of the reactants with the catalyst is difficult due to the relatively high viscosity of the polymer solution, to steric hindrances of the polymer chain, and to the high adsorption of the polymer which, once hydrogenated, tends to remain on the surface of the catalyst interfering with the access to the active centres of the nonhydrogenated polymer. Hence, to achieve complete hydrogenation of the double bonds, large quantities of catalyst and severe reaction conditions are required. Usually this causes decomposition and gelification of the polymer. Furthermore, in the hydrogenation of copolymers of conjugated dienes with vinylaromatic hydrocarbons, the aromatic nuclei are also hydrogenated and it is difficult to effect a selective hydrogenation of the double bonds of the polydiene units. Likewise, the physical separation of the catalyst from the solution of hydrogenated polymer is extremely difficult and in some cases complete elimination is impossible due to the strong adsorption of the polymer on the heterogeneous catalyst.
In processes using Ziegler-type catalytic systems (as mentioned hereinbefore), the reaction takes place substantially in a homogeneous medium and the hydrogenation of the copolymers may be carried out under mild pressure and temperature conditions. Moreover, by adequately selecting the conditions of hydrogenation it is possible to selectively hydrogenate the double bonds of the poly(conjugated diene) blocks without hydrogenating the aromatic rings of the poly(vinylaromatic hydrocarbon) blocks.
Nevertheless, the elimination of the catalyst residues from the reaction product--which is absolutely necessary because these residues have an unfavourable effect on the stability of the hydrogenated polymers--is a complicated and costly step. Other processes using other homogeneous catalysts, e.g. the rhodium compounds described in U.S. Pat. No. 3,898,208 and in Japanese patent JP 01,289,805 have the disadvantage of the high cost of the rhodium catalysts.
It is known that hydrogenation catalysts in which one of the components is a derivative of cyclopentadienyltitanium (U.S. Pat. No. 4,501,857) are used--necessarily in the presence of organolithium compounds--for the hydrogenation of the olefinic double bonds of the polymers of conjugated dienes. European Patent application 0460725 describes the use of a similar catalyst system for the hydrogenation of polymers that had been synthesised by means of an organolithium compound and which have been terminated by the addition of hydrogen, the presence of the lithium hydride formed in the final reaction being necessary in this case to generate an active catalyst. The examples of both publications use the compound bis(cyclopentadienyl) titanium dichloride (Cp.sub.2 TiCl.sub.2).
To obtain more economical hydrogenation processes, present-day industry feels the need of having homogeneous catalysts available which are more effective that those currently known, which are stable and active in concentrations that are sufficiently low so as to be able to avoid the costly step of elimination of catalyst residues from the hydrogenated polymer. Therefore, one object of the present invention is to provide an improved hydrogenation process which will accomplish these goals. It will be appreciated that another object of the present invention is formed by a catalyst composition to be used for said process. As a result of extensive research and experimentation there has been surprisingly found such a catalyst and process aimed at.